1. Field of the Invention
The present invention relates generally to liquid processing systems and methods, and particularly to a method for purifying liquids using a series of reverse osmosis units in which a pressure differential across the membranes of the reverse osmosis units is reduced.
2. Description of the Related Art
Increasing population pressures, expanding industrialization, and various other factors have resulted in increasing demands upon the supply of fresh water in many parts of the world. A number of technologies for purifying water have been developed in response to this need. These technologies primarily remove salt and other impurities from seawater to produce fresh water.
One of the most common technologies used in water (and other liquid) purification is that of reverse osmosis (RO), where a solvent is forced through a permeable membrane from a region of high solute concentration to a region of low solute concentration by applying a pressure greater than osmotic pressure. However, reverse osmosis (RO) technology suffers from certain challenges that include low recovery ratio and membrane scaling. These two challenges control the economics of RO desalination and both of them are related to pressure across the membrane. Higher recoveries can be obtained by increasing the pressure across the membrane. However, such increase in pressure causes (1) higher scale formation that is promoted by compaction and (2) higher salt concentration in the concentrate side of the membrane. In addition, increased pressure across the membranes requires costly membrane assemblies and higher power consumption.
A number of liquid purification systems have been developed in the past. An example of such is found in European Patent Publication No. 1,020,407 published on Jul. 19, 2000 to Uwatech GMBH et al. This reference describes (according to the drawings and English abstract) treating condensate effluent containing ammonium nitrate by successive reverse osmosis to yield a process water stream and a concentrate which is recycled.
Thus, a method for purifying liquids solving the aforementioned problems is desired.